In existing conventional plants for cleaning and bleaching of secondary fibers, such as salvaged old papers and other reused or recycled fiber-containing materials, these materials are collected and segregated in piles according to type, i.e., ledger, newspaper, cardboard, etc. The fiber materials from these piles are blended in a hydropulper, pulped and cleaned, followed when required by a cleaning step carried out by water washing or by flotation. The cleaned and deinked pulp, typically containing from 5 to 10% of the original ink present in the waste paper, is then directed to the bleach plant where it is subjected to one or more bleaching stages, each stage involving distinctly different treating chemicals and process conditions. The initial pulping and washing eliminates most of the contaminants, including ink, present in the fiber, and the subsequent bleaching stages brighten the pulp. The effectiveness of the pulp bleaching is measured and designated by parameters of brightness, contaminant count, and viscosity, which is a measure of pulp strength.
For the cleaning and bleaching of recycled waste paper and other secondary fiber materials, the most common practices employ chlorine-based chemicals, used primarily for fine papers, or hydrosulfite, used primarily for newspaper stock. Various treating sequences are utilized in the bleaching of the recycled pulp to attain desired brightness levels, typically above 76 brightness (GE) and commonly in the range of 78 to over 80% of the fine paper bleaching utilizes a CEH, CH, or H sequence. As commonly employed in the art these letter designations respectively stand for:
C=Chlorination with chlorine (Cl.sub.2) PA1 E=Alkali extraction with NaOH PA1 H=Alkaline hypochlorite (NaOCl) PA1 1) Brightness enhancements or gains, and PA1 2) Increase the optical cleanliness. PA1 disintegrating wastepaper with an alkaline solution to produce an aqueous slurry of pulp fibers, PA1 adding a mixture of alkoxylated lanolin derivatives to the aqueous slurry of pulp fibers to remove the ink components from the waste paper, PA1 forcing air through the slurry so that it foams forming bubbles to which the ink particles adhere, and PA1 removing the foam from the slurry together with the ink particles.
Concern over the negative impact on the environment of chlorine-based bleach plant effluents has accelerated in recent years, particularly since the discovery of the highly toxic chlorinated dioxins and furans in some bleach plant effluents, sludge, and pulp products. Today it is generally accepted that it is critical to reduce the amount of chloro-organics in pulp products and in the plant effluents.
Since formation of chlorinated organics is strongly related to the use and consumption level of molecular chlorine in the chlorination and hypochlorite stages of the conventional bleaching sequence, it is of great importance to significantly reduce the formation of chloro-organics by eliminating the chlorination and hypochlorite stages. Unlike the kraft paper industry (virgin wood processing), the recycled paper industry is only beginning to deal with the issue of chlorinated toxins including the initiation of projects involving the reduction, and preferably elimination, of chlorine and hypochlorite.
Most of the more common treating sequences employed or proposed for bleaching and delignifying of kraft pulp as well as those concerned with bleaching of secondary fibers are chlorine based.
U.S. Pat. No. 4,360,439 discloses a deinking agent composition comprising a salt of a fatty acid, a nonionic surfactant, an anionic surfactant, a sodium carboxycellulose, and an alkaline inorganic salt selected from the group consisting of metasilicate, disilicate, carbonate, borate, and polyphosphate. An improved washing process for deinking printed waste paper is also disclosed.
U.S. Pat. No. 4,390,395 describes a process for disintegrating waste paper to form pulp which comprises disintegrating wastepaper in the presence of an aqueous phase and a peroxide bleaching agent at a high consistency in the range of 22 to 45%. Surfactants are optionally added during the disintegration stage.
A method for the oxygen-alkali treatment of wastepaper containing wet strength resins is disclosed in U.S. Pat. No. 4,416,727. The oxygen-alkali treatment liberates fiber from the resin, and the fiber can be recycled to the papermaking process to make recycled paper products.
The claimed invention is primarily designed to be used in conjunction with the flotation method of deinking, although other methods as well may be employed. A modern deinking flotation system must fulfill two main requirements:
In achieving the main requirements the additional goals for flotation deinking are to accomplish the above but doing so with low solids losses. Additional benefits of flotation deinking also include a reduction of "stickles" content.
Flotation deinking can only remove those ink particles that are referred to as "free"; that is, those ink particles that have been detached or removed from the fibers and are free in the suspension. With some waste papers, the flotation system may be split into what is referred to as pre-dispersion flotation and post-dispersion flotation. Dispersion is a mechanical process that releases additional ink particles from the fibers.
In processing waste paper with the foam flotation method, the stock suspension is comprised mainly of waste, fibers, ash and ink particles. The flotation method is used to selectively separate these ink particles from the suspension. In order to achieve efficient deinking, the ink particles must be separated from the fibers, in other words, they must be free to move in the suspension, and they must be hydrophobic.
In order for a flotation cell to operate correctly and perform its intended function, it is critical that the pulp slurry be prepared properly. Although there are various ways or means available to achieve the desired results, certain guidelines exist. In the case of flotation deinking, the pulp slurry should be of a uniform consistency, generally from 0.7 to 1.5% by weight of pulp depending on the design of the cell. Most flotation cells operate best with a constant feed flow, although some will tolerate a change in flow within a specified range. The pulp slurry should be well screened through small holes, and/or holes and slots in combination, to remove any large particles of debris that would potentially cause operational problems. A minimum amount of ash is also generally needed in the pulp slurry because of its ability in combination with flotation chemicals to enhance the ink removal in a flotation cell.
The chemicals used in the flotation system are of critical importance for the operation of the flotation cell. Types and amounts used typically vary depending on the characteristics of the inks present, the ash content of waste paper, and the location of flotation cells in the deinking system.
The present invention is directed to deinking agents generally referred to as "collector chemicals". As the name implies, collector chemicals are designed to bring together the ink particles that have been freed by the pulping action, such that they can be removed by the air bubbles. For ink removal to occur, the ink particles must come into contact with the collector chemicals, which in turn must come into contact with the air bubbles such that the ink agglomerates can be removed. The formulation of the collector chemicals helps to adjust the surface tension of the air bubbles to achieve this. One of the early downfalls of surfactants used for collector chemicals is that too high a portion thereof was carried over and began to build up into the backwater causing either too great or insufficient foam stability. This problem is detectable from a gradual decrease in brightness over a 36-72 hour period.
Collectors are typically made from naturally occurring materials such as fatty acid soaps; synthetics such as ethylene oxide or propylene oxide alkoxylates; and blends such as ethoxylated fatty acids. Fatty acid soaps are a type of surfactant. One of the most frequently cited reasons for not using fatty acid soaps is that calcium ions must be added. The concerns are not only that another tank and metering system must be added, and that calcium ions are believed to cause scaling and other deposits on the paper machine and within the deinking plant, but that high calcium ion levels contribute to stock loss. Therefore, it is a primary object of this invention to provide a deinking agent and process which provides increased secondary pulp brightness and decreased stock loss levels.
The present invention as disclosed and claimed herein offers an advanced method for the cleaning and brightening of secondary pulps which reduces the amount of subsequent bleaching using chlorine or hypochlorite required to produce recycled paper products having satisfactory brightness.